Process for applying high application-temperature coating to heat-sensitive aluminum alloys

ABSTRACT

A process is provided for coating a given surface of a heat sensitive metal  article (e.g., thermally massive aluminum) with a  high-cure-temperatureating-forming-powder. A first oven heating step elevates the metal article&#39;s temperature to a predetermined preheat cycle high-limit-of-heat-load-temperature. After a first predetermined period of time, the metal alloy article is withdrawn from the oven. A high-cure-temperature-coating-forming-powder is then sprayed onto the given surface of the metal article. The spraying, comprising first and second spray sequences, is initiated essentially immediately after withdrawal of the metal article from the oven. The first spray sequence is a single group of spray passes that applies a coating layer to fill the pores in the given surface with heat melted high-cure-temperature-coating-forming-powder. The second spray sequence builds up the total thickness of coating. A second oven heating step elevates the metal article&#39;s temperature to a predetermined cure cycle high-limit-of-heat-load-temperature for a second predetermined period of time.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for Governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to high application-temperaturecoatings, and more particularly to a process of applying a protective,high-cure-temperature coating to thermally massive heat-sensitivealuminum alloy components without degrading the mechanical properties ofthe components.

(2) Description of the Prior Art

It has long been known and customary in the art to apply protectivecoatings on diverse types of structural substrates. Coating applicationmethods generally include liquid dispersions of synthetic resins viasurface application or immersion, or dry powder coating viaelectrostatic powder spray application. The choice of synthetic resinsfor use as a protective coating is dependent upon both the nature of thesubstrate to be coated and the nature of the intended application of thecoated substrate. For effective protection, the chosen synthetic resinand application method must produce a continuous coating on thesubstrate of sufficient thickness and hardness to isolate the substratefrom the environment(s) from which the substrate requires protection.Aluminum is an excellent selection for a structural material where highstrength-to-weight ratios are desired/required. Heat treated sixthousand (6000), seven thousand (7000) series and A356 aluminum alloys(containing 90-95% aluminum) are used in torpedo applications because oftheir high strength and relatively light weight. Traditionally, however,strength considerations are often given more consideration thancorrosion resistance characteristics during design and materialselection. Accordingly, the designed for strength-to-weight ratios areoften undermined by corrosion when the chosen structural component isexposed to a marine environment for any length of time.

A variety of post-fabrication processes are currently used in an attemptto improve the corrosion resistance of torpedo structural elements madefrom series 6000, 7000 or A356 aluminum alloys. Anodizing, painting andchemical film conversion are the common surface treatments which improvecorrosion resistance by isolating/separating the aluminum alloy from thecorrosive environment. Powder epoxy coating is currently one of the mosteffective and practical surface coating/treatment that can be applied toan aluminum alloy.

The best corrosion resistance performance is generally obtained by usingthe highest-cure-temperature powder epoxies that are currentlyavailable. While small or thin aluminum parts can successfully be powdercoated using standard commercial application techniques, "largethermally-massive" aluminum parts (parts with heavy thick-walledsections) cannot. Current methods of effectively applying suchhigh-cure-temperature coatings to a thermally-massive part require thatthe part to be coated be heated in excess of temperatures at which thealuminum alloy begins to lose tensile/yield strength. Further, partsthat are considered to be thermally massive act as a significant heatsink such that "normal" cold application of powder coatings followed byan oven cure are ineffective. During a cure cycle, the mass of a coldaluminum part draws so much heat away from the coating that it does notallow the coating a chance to flow before curing. A coating film appliedthis way would remain porous, discontinuous and rough. Note that thereis no problem powder coating large steel components (which may be "largethermally-conductive masses") because steel is capable of withstandingmuch higher temperatures without degradation of structural properties.

For the purposes of this description, a "thermally-massive" aluminumcomponent is defined as any component large enough or shaped in such away as to draw heat away from the applied coating during a post spraycure. Some examples of thermally massive aluminum components as theyapply to a torpedo structure are listed in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Examples of Thermally Massive Aluminum Components                             Used in Torpedo Structures                                                                           SECTION OR                                             GENERAL SHAPE/                                                                           APPROX.     WALL        APPROX.                                    DESCRIPTION                                                                              DIMENSIONS  THICKNESS   WEIGHT                                     ______________________________________                                        Ring       12" O.D. ×                                                                          2.0"        10 lbs                                                8" I.D. ×                                                               1.5" long                                                          Hollow Cylinder                                                                          21" diameter ×                                                                      0.35" to    39 lbs                                                12" long    2.12"                                                  Hollow Cone                                                                              21" diameter                                                                              0.3" to     41 lbs                                                tapering to 1.5"                                                              18.5" diameter ×                                                        15.5" long                                                         Hollow Cylinder                                                                          21" diameter ×                                                                      0.3" to     58 lbs                                                12" long    2.0"                                                   Hollow Cylinder                                                                          21" diameter ×                                                                      0.34" to    180 lbs                                               51" long    2.2"                                                   ______________________________________                                    

In general, the components may be large or small, heavy or light, buttypically have thick wall sections. The thick wall sections are slowerto reach oven temperature and draw more heat away from surface appliedcoatings than the thinner sections.

The use of aluminum in torpedo components employs an abnormally largeproportion of aluminum's strength in order to maximize torpedoeffectiveness. Considering that torpedo components may be utilized forpossibly 20 to 30 years, a multiple recoat capability must beaccommodated in the torpedo coating application specification. Duringinitial coating of newly manufactured hardware, recoats are occasionallyrequired due to manufacturing error or other circumstances. Thissituation by itself generally requires the part to be subjected toadditional full coating process cycles. In addition, if and when thecomponent is severely damaged in fleet use and requires recoating, oneor two additional coating cycles may be required at the torpedomaintenance depot. Since most of the torpedo hardware, after theproduction process, will not be tracked with respect to powder coatingcycles, a high recoat capability must be assumed. Based on thesecircumstances, the maximum coating temperatures are generally setslightly lower to allow for multiple cycles. These safety factors havebeen built into the torpedo application specifications to protect thealuminum from excessive heating that could occur during production andsubsequent depot repair.

Accordingly, to date, thermally massive series 6000, 7000 or A356aluminum alloy components have not achieved high performance corrosionresistance characteristics because 1) a lower temperature (i.e., lowerperformance) coating is used, 2) a high-cure-temperature coating is usedbut applied/cured at temperatures that maintain the strength-to-weightratio integrity of the component but that do not provide for sufficientcuring of the coating, 3) a high-cure-temperature coating isapplied/cured at its optimum temperature thereby degrading themechanical properties of the component, or 4) multiple coating cyclesnecessitate the use of low cure temperatures to protect the structuralintegrity of the component.

SUMMARY OF THE INVENTION

Accordingly it is an object of the present invention to provide aprocess for applying a high-cure-temperature coating to heat-sensitivethermally massive aluminum alloy components.

Another object of the present invention is to provide a process ofcoating a heat-sensitive thermally massive aluminum alloy with ahigh-cure-temperature coating that is inert in a marine environment.

Yet another object of the present invention is to provide a process ofcoating a heat treated series 6000, 7000 or A356 aluminum alloy with ahigh-cure-temperature coating such that the coating process does notdegrade the mechanical properties of the aluminum alloy on the initialor subsequent coating applications.

Still another object of the present invention is to provide a process ofcoating a heat treated series 6000, 7000 or A356 aluminum alloy in orderto enhance the alloy's corrosion resistance in a marine environment.

Other objects and advantages of the present invention will become moreobvious hereinafter in the specification and drawings.

In accordance with the present invention, a process is provided forcoating a given surface of a heat sensitive metal article with ahigh-cure-temperature-coating-forming-powder. The process has specialutility and application where the metal article is expected to undergoat least one recoating process during the article's utilization life. Afirst oven heating step, over a first predetermined period of time,elevates the metal article's temperature to a predeterminedpreheat/degas cycle high-limit-of-heat-load-temperature. Thepreheat/degas cycle temperature is a trade-off betweendegree-of-degassification and the desired capability of the metalarticle to retain a predetermined strength characteristic. Thepreheat/degas cycle temperature choice is further made on the basis ofexperimentally or otherwise empirically obtained data. After the firstpredetermined period of time, the metal alloy article is withdrawn fromthe oven. A high-cure-temperature-coating-forming-powder is then sprayedonto the given surface of the metal article. The spraying, typicallycomprising first and second spray sequences, is initiated essentiallyimmediately after withdrawal of the metal article from the oven. Thefirst spray sequence is a group of spray passes that apply a coatinglayer to fill the pores in the given surface with heat meltedhigh-cure-temperature-coating-forming-powder. The second spray sequenceis typically at least one other group of spray passes that build up thetotal thickness of coating resulting from both the first and secondspray sequence to a desired total coating thickness over the givensurface of the metal article. A second oven heating step elevates themetal article's temperature, over a second predetermined period of time,to a predetermined cure cycle high-limit-of-heat-load-temperature. Thecure cycle temperature is based on the desired capability of the metalarticle to retain a predetermined strength characteristic. Theexperimentally or otherwise empirically obtained data includes data thatpredicts the effects of the predetermined preheat and cure cycleshigh-limit-of-heat-load-temperatures, the first predetermined period oftime and the second predetermined period of time. The time andtemperatures are selected such that the mechanical properties of thealloy are not degraded during the coating process.

BRIEF DESCRIPTION OF THE DRAWING(S)

Other objects, features and advantages of the present invention willbecome apparent upon reference to the following description of thepreferred embodiments and to the drawings, wherein:

FIG. 1 is a graph showing a generalized time versus temperatureprogression of the present process; and

FIG. 2 is a graph defining several temperature-time envelopes for a7075-T73 aluminum alloy which, when exceeded in terms of time ortemperature, will potentially result in a greater than 5% loss oftensile/yield strength.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

With reference now to the generalized time versus temperatureprogression (curve 12) in FIG. 1, the process for applying ahigh-cure-temperature coating to a heat-sensitive thermally massivealuminum alloy/component according to the present invention will beexplained. As referred to herein, a high-cure-temperature coating is a250-450° F. cure powder coating where "cure" is defined herein as thechange from a discontinuous particle form to a continuous coating form.The heat-sensitive aluminum alloys referred to herein are series 6000,7000 or A356 aluminum alloys that have undergone a prescribed heattreatment process prior to being coated according to the presentprocess. The process is defined and controlled in order to ensure thatthe mechanical properties of the aluminum alloy are not degraded. Thethree basic steps of the inventive process are defined generally asfollows:

1) Preheat/Degas Cycle,

2) Spray Cycle, and

3) Cure Cycle.

1) Preheat/Degas Cycle

The heat treated aluminum alloy part (or "part" as it will be referredto hereinafter), initially at room temperature, is placed in a preheatedoven (curve segment 12a). As mentioned above, the heat treated aluminumalloy has a known temperature-time envelope above which point theparticular alloy begins to lose its tensile/yield strength. Thistemperature-time envelope is defined by a curve unique to each aluminumalloy. One such set of curves is shown in FIG. 2 for 7075-T73 aluminumalloy undergoing 1, 3 and 10 heating cycles. Time/temperature conditionsbelow each curve are indicative of less than 5% degradation of materialproperties. The preheat/degas cycle is important for three reasons.First, the preheating sets up the coating application as will beexplained further in the description of the spray cycle. Second, thepreheating degasses the part. Third, the preheat/degas cycle shortensthe overall heating cycle time that the part will experience.

With respect to the degassing aspect of this cycle, a higher temperatureis desired in order to successfully degas the part. (Note that if gasremains entrapped in the aluminum when it is coated, the gas may bubblethrough the surface of the coated part when it is reheated during thecure cycle causing porosity of the coating and creating potentialcorrosion sites.) High temperature degassing is especially important forrough surfaces (e.g., surface profiles of 200-1250 microinches) becausethese surfaces will hold more entrapped gas than smoother surfaces.Thus, it is desirable to maintain the temperature of the part within thewindow of temperature defined between the target temperature and maximumallowable part temperature for a prescribed period of time. Theprescribed period is defined and shown in FIG. 1 as the preheat/degassoak cycle (shown as time span line 20). Exact times and temperaturesdepend primarily on the particular aluminum (alloy and heat treatment)and the part size, shape and mass.

2) Spray Cycle

At the conclusion of the preheat/degas soak cycle (line 18), the parthas been heated and degassed and is ready to be coated. Typically, thepowder coating is applied by an electrostatic sprayer, by means ofdipping the part (if it is small enough) in a fluidized bed of thecoating, or by any other acceptable method. For purposes of description,an electrostatic spray method will be assumed. However, it is to beunderstood that the method of applying the coating is not a limitationof the disclosed process.

The part is removed from the oven just prior to beginning the spraycycle. (Practically speaking, the part is removed from the oven beforethe coating is applied due to equipment problems that may result fromtrying to coat the part while it is still in the oven.) By spray coatingthe part while it is still at an elevated temperature, ahigh-cure-temperature coating may be selected such that curing of thecoating will commence during the spray cycle. In addition, if the cooldown period occurring during the spray cycle is minimized, regassing ofthe part is reduced/prevented.

Cooling of the part begins upon removal from the preheat/degas oven(curve segment 12b). Powder spraying passes should begin (dashed line22) and proceed as quickly as possible so that at least a light coatingof powder is applied initially to all surfaces to seal the pores of themetal (dashed line 24). This approach provides two advantages: (1)adhesion is improved because the epoxy is drawn into cooling pores andentrapped within the surface of the aluminum, and (2) upon reheatingduring the curing cycle there is little or no entrapped gasses tore-expand and disturb the melting coating film. Subsequent sprayingpasses can be used to build the desired coating thickness (sprayingcomplete at dashed line 26). Once the pores of the metal are sealedthere is no minimum temperature that must be avoided to preventregassing. In this way, adhesion of the coating is improved whilereducing/eliminating surface imperfections due to the aforementionedproblem of entrapped gas bubbling through the coating during the curecycle.

3) Cure Cycle

At the conclusion of the spray cycle (line 28), e.g., as dictated by thecompletion of the coating application, the part is returned to the ovenand reheated (curve segment 12c). The process of the present inventionprovides for complete "curing" (defined herein as the change from adiscontinuous particle form to a continuous coating form) of the appliedcoating without degrading the mechanical properties of the aluminumalloy. Specifically, the part is re-heated to the target temperature asdefined hereinabove and in FIG. 1. Once the temperature of the part hasreached the target temperature, the temperature thereof and time thereatare monitored as follows. The temperature of the part is maintainedwithin the window of temperature defined between the target and maximumallowable part temperatures. The maximum time for maintaining thetemperature of the part in this window, or cure soak cycle (shown astime span line 30), is selected based upon a combination of theparticular coating's requirements and the particular aluminum alloy. Atthe point in time that the cure soak cycle is completed (line 32), thepart is removed from the oven and is allowed to cool to room temperature(curve segment 12d).

EXAMPLES

The process of the present invention will be illustrated by thefollowing Examples.

EXAMPLE 1: Torpedo Exercise Shell

7075-T73 Aluminum

Weight: approximately 180 lbs

Powder

Powder coating: Farbond LE-3939-G

Manufacturer: Farboil, 8200 Fischer Rd., Baltimore, Md.

Powder Coating Composition:

Epoxy resin (Diglycidylether of Bisphenol A (DGEBA))

Phenolic curing agent

Pigments

Coating Process

Preheat/Degas Cycle Temperature/Time:

Room Temperature to Target Temperature of 325° F. in 50 minutes

Preheat/Degas soak cycle is 325-345° F. for 30 minutes Approximatecomponent temperature after spraying and before entering curing oven:140° F.

Cure Cycle Temperature/Time:

Approximate time to reheat to 300° F.: 30 minutes

Cure Soak Cycle is 300-325° F. for 20 minutes

Maximum recommended coating cycles for this aluminum alloy and thesetime/temperature conditions: 3 cycles

EXAMPLE 2: Torpedo Extender Shell

7075-T7352 Aluminum

Weight: 58 lbs

Powder

Powder coating: 134 Powder Epoxy

Manufacturer: 3M Company, Electronics Products Division, Austin, Tex.

Powder Coating Composition:

Modified epoxy resin (<62% by weight)

Amine curing agent (<37% by weight)

Pigments (<3% by weight)

Coating Process

Preheat/Degas Cycle Temperature/Time:

Room Temperature to Target Temperature of 275° F. in 45 minutes

Preheat/Degas Soak Cycle is 275-290°F. for 25 minutes

Approximate component temperature after spraying and before enteringcuring oven: 190° F.

Cure Cycle Temperature/Time:

Approximate time to reheat to 275° F.: 20 minutes

Cure Soak Cycle is 275-290° F. for 18 minutes

Maximum recommended coating cycles for this aluminum alloy and thesetime/temperature conditions: 10 cycles

EXAMPLE 3: Torpedo Forward Afterbody Shell

A356-T6 Aluminum

Weight: 39 lbs

Powder

Powder coating: Rilsan

Manufacture: Rilsan Corporation, Glen Rock, N.J.

Powder Coating Composition: Nylon 11

Coating Process

Preheat/Degas Cycle Temperature/Time:

Room Temperature to Target Temperature of 390° F. in 40 minutes

Preheat/Degas Soak Cycle is 390-410° F. for 30 minutes

Approximate component temperature after spraying and before enteringcuring oven: 220° F.

Cure Cycle Temperature/Time:

Approximate time to reheat to 390° F.: 30 minutes

Cure Soak Cycle is 390-410° F. for 10 minutes. During the cure cycle forthermoplastic Nylon 11, the discontinuous particles melt and flowtogether.

Maximum recommended coating cycles for this aluminum alloy and thesetime/temperature conditions: 2 cycles

It should be noted from the above examples that various coatings andcure temperature ranges may be used. Depending on the type of aluminumand the cure temperature and time required, the maximum number ofallowable coating cycles is determined. Alternatively, depending on thetype of aluminum and the selected number of allowable coating cycles, apowder material is selected which has the appropriate cure temperatureand time conditions. For example, the experimentally or otherwiseempirically determined curves in FIG. 2 for 7075-T73 aluminum showtime/temperature conditions for 1, 3 and 10 heating cycles.Time/temperature conditions below each curve are indicative of less than5% degradation of material properties. Similar sets of curves have beengenerated for other aluminum alloys (e.g., 7039-T64, 6061-T6, A356-T6)and additional similar sets of curves could be generated for other heatsensitive materials.

The powder coating process cycle for thermally massive aluminum partsconsists of a preheat/degas cycle, one spray application cycle, and onecure cycle. Parameters that are controlled during each powder coatingheat cycle are: 1) overall cycle times, 2) cumulative time above targettemperature, and 3) maximum allowable temperature.

1) Overall oven cycle time consists of two parts: up to two hours ofoven time for preheat/degas, and up to two hours of oven time for cure.

2) Cumulative time above target temperature is the total timeaccumulated during both the preheat/degas soak and cure soak cycles,i.e., when the part is above the target temperature. The amount of timeeach part spends above target temperature during the preheat/degas soakcycle and cure soak cycle may be adjusted by the coater as dictated bythe mass and other specific properties and requirements of the part. Thecumulative time should generally not exceed 1 hour.

3) Maximum cure temperatures and maximum time at cure temperature limitsare provided to ensure that excessive temperatures or times which mightdeteriorate mechanical properties of the aluminum will not be used.These maximums allow a sufficient safety factor for limited futurecoatings of the same part.

The advantages of the present invention are numerous. Heat treatedthermally massive aluminum alloys may be coated with a high performance,corrosion resistant coating that is inert in a marine environment.Furthermore, the controlled coating process is designed to maintain themechanical properties of the non-coated aluminum alloy. For the Examplesdescribed herein, tensile and yield strengths of the coated parts weremaintained within 5% of the original specified values for the aluminumalloys.

While the present invention has been described relative to a preferredembodiment, it is not so limited. The controlled process may be adaptedto a variety of coating/heat sensitive substrates by merely adjustingthe time/temperature specifications to correspond with a coating andsubstrate. Thus, it will be understood that many additional changes inthe details, materials, steps and arrangement of parts, which have beenherein described and illustrated in order to explain the nature of theinvention, may be made by those skilled in the art within the principleand scope of the invention as expressed in the appended claims.

What is claimed is:
 1. A process of coating a given surface of a heatsensitive metal article with a curable powder coating, comprising thesteps:elevating the metal article's temperature in an oven to a selectedtemperature; maintaining the metal article at approximately saidselected temperature for a first period of time: withdrawing the metalarticle from the oven; spraying said curable powder coating onto saidgiven surface of the metal article, said spraying being initiatedessentially immediately after withdrawal of the metal article from theoven, said spraying comprising first and second spray sequences, saidfirst spray sequence applying a coating layer having a thickness chosento be adequate to fill pores in said given surface with said curablepowder coating, said second spray sequence building up a total thicknessof said curable powder coating resulting from both the first and secondspray sequence to a total coating thickness over said given surface ofthe metal article; re-elevating the metal article's temperature in saidoven to said selected temperature; maintaining the metal article atapproximately said selected temperature for a second period of time;said selected temperature, said first period of time, and said secondperiod of time being chosen based upon experimentally determined datawhich defines a curve plotting temperature versus time of heating forwhich the metal article retains a percentage of its nonheated tensileand yield strength characteristics after undergoing a plurality ofheating/cooling processes involved in at least two individual instantprocesses of coating: said selected temperature being chosen as atemperature value on said curve; and the first and second periods oftime being so chosen that the cumulative time consisting of said firstand second periods of time does not exceed the time of heating on saidcurve corresponding to said selected temperature.